A switchable macrocycle-clip complex that functions as a NOR logic gate

Article abstract of DOI:10.1039/b417823h

We have synthesized a new molecular switch-based on a macrocycle-clip complex-whose switching behavior not only can be controlled through the use of either K⁺-[2,2,2]cryptand or NH₄⁺-Et ₃N systems but also provides color changes that are visible to the naked eye; consequently, this system operates as a two-input NOR functioning molecular logic gate. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b417823h

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A switchable macrocycle–clip complex that functions as a NOR logic
gate{
Pin-Nan Cheng, Pinn-Tsong Chiang and Sheng-Hsien Chiu*
Received (in Cambridge, UK) 25th November 2004, Accepted 4th January 2005
First published as an Advance Article on the web 21st January 2005
DOI: 10.1039/b417823h
a molecular clip. We anticipated that the complementary electronic
nature of the TTF and pyridinium motifs, the good separation
˚
(6.7 A) for p-stacking of a pyridinium moiety between the two
We have synthesized a new molecular switch—based on a
macrocycle–clip complex—whose switching behavior not only
can be controlled through the use of either K+–[2,2,2]cryptand
+
parallel aromatic side walls of the glycoluril-based molecular clip,
and p-stacking interactions between the non-threaded ‘‘exo’’
TTF arm of clip 2 and the pyridinium rings of the macrocycle
1?2PF6 would all assist molecular clip 2 to complex with 1?2PF6
with reasonable affinity. The four triethylene glycol chains of clip
2 are present not only to increase its solubility but also to allow
or NH4 –Et3N systems but also provides color changes that are
visible to the naked eye; consequently, this system operates as a
two-input NOR functioning molecular logic gate.
Although many elegant pseudorotaxane-like1 molecular switches2
have been developed in recent years, the number of recognition
motifs that can be exploited for the preparation of these complexes
remains limited because of structural complexity and/or weak
intermolecular interactions. Thus, we became interested in
preparing new supramolecular assemblies by taking advantage of
the increased binding affinities that clip-shaped guest molecules
exert toward their macrocyclic hosts. Although many molecular
tweezers3 and clips4 have been prepared as effective synthetic
receptors, there are only a few examples of molecular clips that
form complexes with macrocycles;5 of these macrocycle–clip
complexes, we are unaware of any that can be switched between
their threaded and unthreaded states by the application of external
stimuli.6 The addition and removal of metal ions6b,7 and
protons6b,8 are efficient methods for switching supramolecular
systems between different states; such systems that can be switched
in two different ways and exhibit detectable gains or losses of the
optical signals of their different states may be considered as
molecular logic gates.9 In this paper, we report a new molecular
switch—based on a macrocycle–clip complex—whose switching
behavior not only can be controlled through the use of either K+–
…
[C–H O] hydrogen bonds to form between the pyridinium
a-protons and the oxygen atoms of these triethylene glycol side
chains, which are interactions that may partially decrease the free
energy of binding during the complexation process.
Although clip 2 is only moderately soluble in MeCN, the
addition of macrocycle 1?2PF6 to the solution helped to dissolve
any remaining insoluble clip 2; more interestingly, this process
+
[2,2,2]cryptand or NH4 –Et3N systems but also provides color
changes that are visible to the naked eye; consequently, this system
operates as a two-input NOR10 functioning molecular logic gate.
We synthesized macrocycle 1?2PF6 in five steps from 2,49-dibro-
moacetophenone. The structure of 1?2PF6 is reminiscent of a
combination of a ring-expanded [18]crown-6 (18C6) unit—which
we expected to bind to alkali metal ions—and a p-electron-
deficient aromatic system, which we expected would bind
p-electron-rich units, such as tetrathiafulvalene (TTF) (Scheme 1).
Because of its flexible molecular structure, relative to those of 18C6
itself and other effective cyclophane-based p-electron acceptors,11
we expected that 1?2PF6 would have quite low affinities for both
alkali metal ions and p-electron-rich aromatic rings. Thus, we
synthesized the glycoluril-based compound 2 in which two TTF
molecules are positioned parallel to each other as the side walls of
{ Electronic supplementary information (ESI) available: Experimental
procedures for the preparation of 1?2PF6, 2, and 3?2PF6 and their
characterization data. See http://www.rsc.org/suppdata/cc/b4/b417823h/
*shchiu@ntu.edu.tw
Scheme 1 Schematic representation of the complexation and switching
behavior of the macrocycle–clip complex.
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excess amount of KPF6. This observation suggests that the 18C6-
like motif of macrocycle 1?2PF6 is likely to be the one that
recognizes the K+ ions; the conformational changes that this
binding event causes the aromatic units of macrocycle 1?2PF6 to
undergo may be the source of the negative allosteric effect.13
To remove the complexed K+ ions from macrocycle 1?2PF6, we
added an excess of a very strong binder, [2,2,2]cryptand, to the
solution. As we anticipated, the puce color of the solution returned
instantly (Fig. 1A-c) upon the addition of [2,2,2]cryptand. The 1H
NMR spectrum (Fig. 2c) of this mixture appears to be similar to
that of the original solution of macrocycle 1?2PF6 and clip 2
(Fig. 2a); this observation implies that the original [(1,2)?2PF6]
complex has regenerated in the solution. This threading–
unthreading process can be repeated several times by adding
excess amounts of K+ ion and [2,2,2]cryptand sequentially; the
reversible color changes allow us to monitor the switching process
by the naked eye.
Fig. 1 (A) A photograph depicting the color changes (MeCN, 3 mM,
298 K) that occur in the switching process and (B) the corresponding
partial UV-vis spectra (MeCN, 1.5 mM, 298 K) of (a) an equimolar
mixture of 1?2PF6 and 2; (b) the mixture obtained after adding KPF6
(20 equiv.) to solution (a); (c) the mixture obtained after adding
[2,2,2]cryptand (20 equiv.) to solution (b); (d) the mixture obtained after
adding NH4PF6 (20 equiv.) to solution (a); (e) the mixture obtained after
adding Et3N (20 equiv.) to solution (d).
immediately changed the color of the solution from light-yellow to
puce (Fig. 1A-a). The UV spectrum of the complex formed
between 1?2PF6 and 2 in MeCN (Fig. 1B-a) displays a charge
transfer band at 533 nm, which suggests the formation of the
macrocycle–clip complex [(1,2)?2PF6]. A Job plot based on this
absorption in MeCN affords conclusive evidence for 1 : 1
Alternatively, this [2]pseudorotaxane-like macrocycle–clip mole-
cular switch can be switched by the sequential addition of
+
NH4 ions and base. Adding ammonium hexafluorophosphate
(NH4PF6) to the puce solution of macrocycle 1?2PF6 and clip 2 in
MeCN also causes the color of the solution to turn light-yellow
+
1
(Fig. 1A-d). The similar sizes and binding affinities of NH4 and
complexation (see supporting information{). From a H NMR
spectroscopic dilution experiment, we determined the binding
K+ ions toward 18C6 may explain these phenomena.14 Adding an
excess of Et3N to the mixture switches the color of the solution
back to puce immediately (Fig. 1A-e). The addition of the base
leads to the formation of NH3 and the Et3NH+ ion, neither of
which binds strongly to the 18C6 motif of 1?2PF6, and, thus, the
threading of clip 2 through the macrocycle regenerates the complex
[(1,2)?2PF6]. Again, the switching process can be visualized by
observing the color of the solution of the macrocycle–clip complex,
which can also be operated reversibly when adding NH4PF6 and
Et3N sequentially.
constant between macrocycle 1?2PF6 and molecular clip 2 in
CD3CN to be 4900 ¡ 180 M21 12
. Adding 20 equiv. of potassium
hexafluorophosphate (KPF6) to the puce solution of a mixture of
1?2PF6 and 2 in MeCN switched the color of the solution back to
its original yellow hue (Fig. 1A-b). The significant decrease in the
intensity of the charge transfer band in the UV-vis spectrum
(Fig. 1B-b) and the appearance of the characteristic absorptions of
the free clip 2 (Fig. 2b) in the 1H NMR spectrum both suggest that
dissociation of the complex [(1,2)?2PF6] occurred. The macro-
cycle 3?2PF6, whose structure differs from that of 1?2PF6 by the
absence of the carbonyl groups, also generates a puce solution
when mixed with clip 2, but the color of the solution of
[(3,2)?2PF6] did not switch back to light yellow upon adding an
As anticipated, the addition of both KPF6 and NH4PF6 to the
mixture of 1?2PF6 and 2 also switches the color of the solution
from puce to light yellow. Thus, if we consider the intensity of
the absorbance at 533 nm in the UV-vis spectrum of complex
+
[(1,2)?2PF6] as the output and the K+ and NH4 ions as the
inputs, the switching of the macrocycle–clip system reflects the
operation of a two-input NOR logic gate.
We have prepared a new macrocycle–clip complex that
functions as a molecular switch. Both K+–[2,2,2]cryptand and
+
NH4 –Et3N stimuli control the movement of this molecular switch
between its threaded and unthreaded states.15 The color changes,
which are observable to the naked eye during switching by either
of these two different methods, allows the macrocycle–clip
complex system to function as a two-input molecular NOR
logic gate.
This work was supported by the National Science Council,
Taiwan (NSC-93-2113-M-002-020).
Pin-Nan Cheng, Pinn-Tsong Chiang and Sheng-Hsien Chiu*
Department of Chemistry, National Taiwan University, Taipei, Taiwan,
10617, ROC. E-mail: shchiu@ntu.edu.tw; Fax: +886 2 24980963;
Tel: +886 2 223690152 ext. 150
Fig. 2 Partial 1H NMR spectra (400 MHz, CD3CN, 298 K) of (a) a
mixture of 1?2PF6 and 2 (10 mM each); (b) the mixture obtained after
adding KPF6 (20 equiv.) to the solution in (a); and (c) the mixture
obtained after adding [2,2,2]cryptand (20 equiv.) to the solution in (b).
Notes and references
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Chem. Commun., 2005, 1285–1287 | 1287